It is common practice to manufacture plastic film in the following manner. First, plastic in molten--i.e. liquid and semi-liquid--form is extruded through an annular nozzle. The resulting bubble or tube of liquid is then subjected to an internal air pressure, which inflates the tube and causes it to balloon out to many times the diameter of the annular nozzle. The inflated tube cools and cures, and is gathered up on rollers.
The economics of the process gives rise to a compromise or trade-off between the quality of the extruded film and the speed at which the film is produced. If the film passes through the nozzle too quickly, the surface texture starts to acquire the so-called "sharkskin" quality. This problem is also referred to as "melt-fracture".
One way of alleviating the sharkskin effect is to increase the annular width of the nozzle gap. However, if the nozzle gap is wide, the film has to be stretched or inflated even more to get the thickness of the film down to the desired value. The more the tube is inflated, the more likely it is that the film will be uneven as to its thickness and also will have other inferior properties.
The invention is aimed at increasing the speed at which plastic film with good properties can be produced, while avoiding the difficulties of a sharkskin surface, and of avoiding also the difficulties of the variations in the thickness etc. of the film that would follow from increasing the width of the nozzle.